One possible reason why the space key did not respond when other keys were pressed at the same time is that your keyboard has a limited key rollover. Key rollover is the ability of a keyboard to register multiple key presses simultaneously. Some keyboards have a low key rollover, which means they can only handle a few keys being pressed at once. If you press more keys than the keyboard can handle, some of them will be ignored or cause ghosting (false inputs).
To test your keyboard’s key rollover, you can use this online tool: https://joltfly.com/key-rollover-test/. It will show you how many keys your keyboard can register at the same time and which keys are affected by ghosting.
If your keyboard has a low key rollover, you can try to change the space key to another key that is less likely to conflict with other keys, such as Shift or Ctrl. Alternatively, you can use a keyboard that has a higher key rollover, such as a mechanical keyboard or a gaming keyboard.
Another possible reason why the space key did not respond is that there is a bug in your code or in Unity’s Input system. To debug your code, you can use Debug.Log statements to print out the values of Input.GetKeyDown for each key and see if they match your expectations. You can also use breakpoints and step through your code to see where it fails. To debug Unity’s Input system, you can use the Input Debugger window (Window > Analysis > Input Debugger) to inspect the state of all input devices and events.